Gather all ingredients.
Melt butter in a large soup pot over medium-high heat. Add mushrooms and 1 pinch salt; cook until mushrooms release their juices, 5 to 10 minutes. Reduce heat to medium-low; continue cooking, stirring often, until juices evaporate and mushrooms are caramelized, 15 to 25 minutes. Set aside a few attractive mushroom slices for garnish later, if desired.
Add onion to mushrooms; cook and stir until onion is soft and translucent, about 5 minutes.
Stir in flour; cook, stirring often, to remove raw flour taste, about 2 minutes.
Pour in 4 cups chicken broth and 1 cup water; add thyme bundle and garlic cloves. Reduce heat to low; simmer for 1 hour. Remove and discard thyme bundle.
Fill blender halfway with soup. Cover and hold lid down with a potholder; pulse a few times before leaving on to blend until smooth and thick. Pour into a pot. Repeat with remaining soup.
Stir in cream; if too thick, add a little chicken broth or water. Season with salt and black pepper.
Divide soup among bowls; garnish with reserved mushroom slices and thyme leaves.
